The present disclosure relates to a developing apparatus that is used for image forming apparatuses such as a copy machine, a printer, a facsimile, a multi-function machine of them and the like that use an electro-photographic system, and an image forming apparatus that includes the developing apparatus, more particularly, to a developing apparatus that performs supply of a two-component developer including a toner and a carrier, and discharges a surplus developer, and to an image forming apparatus that includes the developing apparatus.
In an image forming apparatus, a latent image formed on an image bearing member that includes a photosensitive drum and the like is developed by a developing apparatus to visualize the latent image as a toner image. As one of such developing apparatuses, a two-component developing system, which uses a two-component developer, is employed. This kind of developing apparatus stores, in a developer container, a developer including toners and carriers, disposes a development roller that supplies the developer to the image bearing member, and disposes a stir member that supplies the developer to the development roller stored in an inside of the developer container while conveying and stirring the developer.
In this developing apparatus, the toners are consumed by a development operation, while the carriers are not consumed and remain in the developing apparatus. Accordingly, the carriers stirred in the developer container together with the toners are deteriorated as the stirring frequency increases, as a result of this, charge performance of the carriers for the toners gradually becomes low.
Because of this, a developing apparatus is proposed, which supplies, into a developer container, a developer including carriers and discharges a surplus developer, thereby curbing deterioration in charge performance.
For example, a structure is known, in which two stir members, which each include a rotational shaft and a spiral blade spirally formed on the rotational shaft, are disposed in parallel with each other in respective convey chambers. A partition portion is disposed between the convey chambers, and at both end portions of the partition portion, communication portions for moving the developer are disposed. And, a developer discharge opening is formed in a downstream in a developer convey direction with respect to one of the convey chamber, and between the stir member and the developer discharge opening, a reverse spiral blade, which is spirally formed in a direction opposite to a direction of the spiral blade of the stir member, is disposed as a restriction portion integrally with the rotational shaft.
According to the above structure, when the developer is supplied into the developer container, thanks to rotation of the stir member, the developer is stirred and conveyed to the downstream side of the convey chamber. When the reverse spiral blade rotates in the same direction as the stir member, a convey force in a direction opposite to the developer convey direction by the stir member is given to the developer by the reverse spiral blade. The developer is blocked in the downstream side of the convey chamber by the convey force in the opposite direction, whereby the developer is increased in height level, so that a surplus developer goes over the reverse spiral blade (restriction portion), moves to the developer discharge opening to be discharged to outside of the developer container.
Besides, in a case where a two-component developer is used, to supply toners by an amount of toners consumed for development, it is necessary to measure a toner concentration in a developer by means of a toner concentration sensor that is disposed in a developer container. For example, a structure is known, in which a toner concentration sensor is disposed on a bottom portion of a housing that stores a two-component developer.
On the other hand, in the above image forming apparatus, a method is known, in which to secure a sufficient fixing performance and improve image quality in a case where especially a recording medium is thick in thickness, a speed (process speed), at which the recording medium on which a toner image is transferred passes through a fixing apparatus, is lowered. Here, if drive sources of the fixing apparatus and the developing apparatus are the same as each other, the developing apparatus also is changed to the low speed drive, so that the convey speed of the developer in the developing apparatus rapidly becomes low and the developer amount in the developing apparatus dramatically changes.
As a result of this, immediately after the process speed is changed, the developer amount in the developing apparatus is not an amount suitable for the process speed, accordingly, an excessive stress acts on the developer and a phenomenon occurs in which the toner concentration in the developing apparatus does not rise, whereby an image concentration becomes low. Besides, if the developer amount dramatically reduces, a corrugation occurs on a developer surface along an outer shape of the stir member in the developing apparatus, whereby there is a disadvantage that if an image is output in the state, an image unevenness occurs. Especially, in a fast speed machine, if the process speed is decelerated at a constant rate, a speed difference becomes large between the usual time and the deceleration time, so that the above disadvantage remarkably occurs.
As a developing apparatus that employs a method for stabilizing a discharge amount of a surplus developer from the developing apparatus, a developing apparatus is known, in which a shutter member is disposed at a developer discharge opening, during only a drive time of the developing apparatus or only a developer supply time, the developer discharge opening is opened. Besides, an image forming apparatus is known, which predicts a change of the developer storage amount in the developing apparatus due to a change of environmental conditions, and in accordance with the prediction result, controls a rotational speed of a stir member.
In the structure in which the two stir members, which each include the rotational shaft and the spiral blade spirally formed on the outer circumference of the rotational shaft, are disposed in parallel with each other in the respective convey chambers, even during a time an additional developer is not supplied, the developer conveyed by the spiral blade of the stir member moves to the downstream side of the convey chamber like a wave along an outer circumference of the spiral blade and collides with the restriction portion. When the developer collides with the restriction portion, the developer height level differs with respect to an outer circumference of the restriction portion in accordance with an axis-directional position of the spiral blade with respect to the restriction portion.
If the developer collides with the restriction portion at a high position of the developer height level (developer surface), thanks to power of the collision, the developer goes over the restriction portion to move to the developer discharge opening, so that there is a risk that the developer is excessively discharged and the developer amount in the developer container does not stabilize. Besides, there also is a risk that spattering of the developer occurs before the restriction portion. Especially, in an image forming apparatus which forms an image at a high speed, the stir member rotates at a high speed together with a photosensitive drum, so that a disadvantage remarkably occurs, in which the developer is excessively discharged and the spattering occurs. To prevent such disadvantages, it becomes necessary to slow down the speed of the developer sent to the restriction portion and to make the change of the developer surface moderate.
On the other hand, to accurately detect a toner concentration by means of a toner concentration sensor, it is necessary to make the developer sufficiently present in an entire detection area of the toner concentration sensor. Because of this, in a case where the toner concentration sensor is disposed on a bottom surface of a developer container, it is necessary to make the developer stay at a sensor disposition position.
Accordingly, to curb the change of the developer surface and increase the detection accuracy of the toner concentration sensor, it becomes necessary to make the developer stay at two positions, that is, the sensor disposition position and a position close to the restriction portion in an upstream with respect to the restriction portion. However, if the developer is made to stay at two positions in a circulation route of the developer, it becomes hard to maintain a circulation balance of the developer. Especially, in the developing apparatus which improves the image quality by changing the process speed between an image forming time and a fixing time and conveying a recording medium at a low speed during the fixing time, when the process speed is changed and the rotational speed of the stir and convey member also is changed, the circulation balance of the developer is lost, and in the case where there are two staying positions for the developer, correction becomes hard.
Besides, according to the method in which the shutter member is disposed at the developer discharge opening, and the developer discharge opening is opened during only a drive time of the developing apparatus or only a developer supply time, it is necessary to dispose the shutter member and its drive mechanism, and open-close control of the shutter member also becomes necessary. Besides, according to the method which predicts a change of the developer stored amount in the developing apparatus due to a change of environmental conditions, and in accordance with the prediction result, controls the rotational speed of the stir member, the rotational speed of the stir member is controlled in accordance with the environmental conditions, so that it is necessary to independently drive the stir member. Accordingly, structures of both the apparatuses become complicated, which leads to cost increase.